Manufacture of piezoresistive bars

ABSTRACT

A method of preparing multiple bars from an elongated cylinder of piezoresistive material, which results in each bar, in its finally prepared state, being releasably joined to another bar which has come from an adjacent region in the cylinder.

KP? 3,685,110 [151 3,685,1 10 Randolph, 7/ ms c {451 Aug. 22, 1972 MAN]935 Klingsporn ..3l0/9.5 X BARS .956 Crooks et al. ..29/25.35 InventorGeorge Randolph, R R. 1, ii ifiii $33 $3227.31:11111113111113234233 Box391, Corvallis, Oreg. 97330 [22] Filed: Aug. 31, 1970 PrimaryExaminer-John F. Campbell Assistant Examiner-Carl E. Hall [21] Appl'68l65 Attorney-Kolisch & Hartwell 52 US. Cl. ..29/25.35, 29/423, 29/424,ABSTRACT 4 310/9, 310/95 A method of preparing multiple bars from anelon- Int. Clgated cylinder of piezoresisflve material results 0f eachbar prepared state 57 156/264 releasably joined to another bar which hascome from an adjacent region in the cylinder.

[56] References Cited 5 Claim, 5 Drawing Figures I UNITED STATES PATENTS2,332,447 10/ 1943 Higgins 156/264 X 36851? 1 UR IN 2 i25a35 PATENTEUws22 m2 INVENTOR MM 4. M.

HHg S.

MANUFACTURE or PIEZO isTivE BACKGROUND OF THE INVENTION This inventionpertains to the preparation of piezoresistive bars suitable for use invarious kinds of electromechanical applications. More particularly, itpertains to a method of obtaining from a unitary mass of such material,groups (two or more) of bars having closely matching electrical andmechanical charactenstics.

There are many applications today where piezoresistive bars are used.For example, various types of deflection-sensing devices and the likemay employ such has as mechanical-to-electrical transducers. Frequently,it is desirable that matched pairs (or a larger group) of bars be usedin order to maximize accuracy in the operation of a device. The termmatched herein is used in the sense that the bars in a particular pairor group have substantially the same electrical and mechanicalcharacteristics. In the past, however, the obtaining of matched bars hasbeen quite costly, since it has usually required time-consumingcomparative testing (after manufacturing) of different bars to find onesthat are similar. Manufacturing practices heretofore have notincorporated satisfactory techniques for producing pairs (or largergroups) of matched bars which avoid such comparative testing procedures.

SUMMARY OF THE INVENTION A general object of the present invention,therefore, is to provide a novel method of producing piezoresistive barswhich takes care of the problem of obtaining matched bars in a practicaland satisfactory manner.

More specifically, an object of the invention is to provide a novelmethod of dividing a mass of piezoresistive material into multiple bars,or units, of smaller dimensions, with bars which come from adjacent regions in the mass maintained in close proximity to one anotherthroughout the production process. Experience has shown that two or morebars coming from adjacent regions in such a parent mass of materialexhibit substantially the same electrical and mechanical properties. Bymaintaining each bar (during the manufacturing process) closely adjacentanother bar which has come from substantially the same region in theoriginal mass, the guesswork encountered heretofore in obtaining matchedbars is avoided. In other words, the bars which are held in closeproximity, as contemplated herein, during the preparation process fromthe mass are assured of having similar, and in most cases, substantiallythe same electrical and mechanical characteristics.

According to a preferred manner of practicing the invention, theproposed method comprises the steps of dividing a mass into one set ofsubstantially planar and generally parallel slabs, releasably adheringsuch slabs with a dissolvable adhering agency to form an assemblywherein the slabs have substantially the same positions relative to eachother that they had in the original mass, and then dividing the assemblyalong planes disposed at angles to the planes of the first-mentionedslabs to form another set of substantially planar slabs. It will beapparent that each slab in the second-mentioned set will containadjacent bars, releasably joined through the adhering agency. Further,it will be apparent that adjacent bars in such a slab are ones whichhave come from adjacent regions in the original mass. Thus, merely byselecting two or more adjacent bars in such a slab, a user is assured ofobtaining bars having closely matching characteristics.

DESCRIPTION OF THE DRAWINGS These and other objects and advantagesattained by the invention will become more fully apparent as thedescription which follows is read in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a fragmentary perspective view of an elongated cylinder ofpiezoresistive material suitable as a parent for the preparation of barsas contemplated herein;

FIG. 2 is a perspective view of the cylinder of FIG. 1 divided into oneset of substantially planar and generally parallel slabs;

FIG. 3 is a perspective view illustrating an assembly ascontemplatedherein formed by releasably adhering the separate slabs shown in FIG. 2.

FIG. 4 is a perspective view illustrating the assembly of FIG. 3 dividedinto another set of slabs along planes which are substantially at rightangles to the planes of the first-prepared slabs; and

FIG. 5 is a perspective view illustrating the bars which result in oneof the slabs of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION Indicated generally at 10 in FIG.1 is an elongated solid cylinder, or mass, of a piezoresistive material,such as a suitable P-type silicon. The particular cylinder shown has adiameter of about 1 inch, and an axial length of about 0.4 inches.Cylinder 10 may, for example, comprise a cut-off section of aconsiderably longer cylindrical rod, in which form silicon of the typegenerally described above is often manufactured.

Cylinder 10, it should be understood, is merely illustrative of one typeof mass with which the present invention may be practiced. Put anotherway, neither the configuration nor the dimensions of the mass arecritical, so long as the selected mass is large enough to permit itseconomical division (as will be more fully described) into bars of thesize desired, and so long as it has one dimension at least equaling thelongest dimension desired in the final bars. In the specific practice ofthe invention illustrated and described herein, bars are to be preparedwhich have a length of about 0.4 inches, and a substantially squaretransverse cross-sectional configuration with each side about 0.004inches. It should be noted that while particular dimensions arementionedherein by way of illustration, the relative sizes of piecesshown in the drawings are intentionally distorted in order to obtainbetter clarity therein.

According to the invention, cylinder 10 is first divided, preferably byslicing it, axially as shown in FIG. 2 along spaced substantiallyparallel planes to produce a first set of slabs 12. Each slab l2 hereinhas a substantially uniform thickness of about 0.004 inches. Thedividing operation, by slicing or otherwise, may of course be performedin any one of a number of conventional ways.

After preparation of slabs 12, the same are joined as indicated in FIG.3 to form a unitary assembly 14. As

contemplated herein, such joining is accomplished through adheringadjacent ones with a suitable dissolvable adhering medium, or agency,such as a dissolvable cement. There are numerous types of such cementwhich will perform satisfactorily for this purpose, and the particularone employed is a matter of choice. It should be noted that the slabs inassembly 14 are separated by and joined through layers 13 of such acement, and have substantially the same positions relative to each otherthat they had in cylinder 10.

Referring to FIG. 4 in the drawings, after formation of assembly 14, thesame is divided, preferably by slicing, as indicated in the figure. Morespecifically, the assembly is divided along multiple spacedsubstantially parallel planes which, in the particular case beingdescribed, are substantially at right angles to the planes of slabs 12.This dividing operation may be performed in the same manner as the onepreviously described. What results from this operation is another set ofslabs 16.

Referring to FIG. 5, one of slabs 16 is shown therein. It will be notedthat a slab 16 contains a plurality of elongated bars, or units, such asbars 18, with adjacent bars joined through a portion of a layer of thecement previously mentioned. And, it will be apparent that adjacent barsin a slab 16 are ones which have come from adjacent regions in cylinder10.

What results, therefore, from the various steps described herein areslabs containing multiple adjacent bars which have come from adjacentregions in the original mass from which they have been prepared, andwhich, therefore, have, to a high degree of probability, substantiallythe same electrical and mechanical characteristics. When it is desiredto separate two or more matched bars for use, a slab 16 may be placed ina suitable solvent for the particular cement used, with the various barsin the slab then being freed. In any suitable manner, after freeing ofthe bars from he slab, groups of two or more adjacent bars may beseparated and stored for use. Bars which do not have the desiredcross-sectional configurations, by virtue of their having come fromadjacent the outside regions of the original mass, may be discarded.

The method described herein, therefore, proposes a simple and reliableway of introducing into the steps of manufacturing piezoresistive barstechniques for greatly simplifying the selection of groups of matchedbars. More specifically, the method of the invention contemplates thatthe finally prepared bars, prior to their separation into individualunits, are held together in groups, (i.e., such as in slabs 16) whereinadjacent bars are assured to have come from adjacent regions in theoriginal mass from which they have been produced. The resulting slabscontaining multiple bars are convenient to handle, and minimize thechance of the individual bars inadvertently becoming separated.

It will be apparent to those skilled in the art that the steps of theinvention may readily be practiced on masses of various differentconfigurations and sizes to produce different shapes and sizes of bars.Thus, while a preferred method of practicing the invention has beendescribed herein, it is appreciated that variations and modificationsmay be made without departing from the spirit of the invention.

tis laim d d ired tos cureb Lette P tent: A? metliod of pi' eparing froma inass o piezoresistive material multiple smaller units of suchmaterial, and of positively maintaining in close proximity to eachother, during and throughout such preparing, units which come fromadjacent regions in the mass, said method comprising dividing said massinto one set of substantially planar and generally parallel slabs,

releasably joining said slabs to form an assembly wherein the slabs havesubstantially the same positions relative to each other that they had inthe mass,

dividing said assembly along planes disposed at angles to the planes ofsaid first-mentioned slabs to form another set of substantially planarslabs, each containing plural adjacent releasably joined separableunits, and

until separating of adjacent units in said second-mentioned slabs,preserving the spacial and configurational relationships of such unitsin such slabs.

2. The method of claim 1, wherein said joining is accomplished throughadhering adjacent ones of said first-mentioned slabs with a dissolvableadhering agency.

3. The method of claim 1, wherein said first-mentioned dividing step isperformed along one set of substantially parallel planes, and saidsecond-mentioned dividing step is performed along another set of planeswhich are substantially at right angles to the planes of saidfirst-mentioned slabs.

4. The method of claim 1, wherein said dividing steps are performed byslicing operations.

5. The method of claim 1, wherein said mass comprises an elongatedcylinder, said first-mentioned dividing step is performed generallylengthwise of said cylinder and said second-mentioned dividing step isperformed generally lengthwise of said assembly.

1. A method of preparing from a mass of piezoresistive material multiplesmaller units of such material, and of positively maintaining in closeproximity to each other, during and throughout such preparing, unitswhich come from adjacent regions in the mass, said method comprisingdividing said mass into one set of substantially planar and generallyparallel slabs, releasably joining said slabs to form an assemblywherein the slabs have substantially the same positions relative to eachother that they had in the mass, dividing said assembly along planesdisposed at angles to the planes of said first-mentioned slabs to formanother set of substantially planar slabs, each containing pluraladjacent releasably joined separable units, and until separating ofadjacent units in said second-mentioned slabs, preserving the spacialand configurational relationships of such units in such slabs.
 2. Themethod of claim 1, wherein said joining is accomplished through adheringadjacent ones of said first-mentioned slabs with a dissolvable adheringagency.
 3. The method of claim 1, wherein said first-mentioned dividingstep is performed along one set of substantially parallel planes, andsaid second-mentioned dividing step is performed along another set ofplanes which are substantially at right angles to the planes oF saidfirst-mentioned slabs.
 4. The method of claim 1, wherein said dividingsteps are performed by slicing operations.
 5. The method of claim 1,wherein said mass comprises an elongated cylinder, said first-mentioneddividing step is performed generally lengthwise of said cylinder andsaid second-mentioned dividing step is performed generally lengthwise ofsaid assembly.